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Sunday, 20 October 2019

Intrinsic vs Extrinsic Semiconductors

Intrinsic Semi Conductors:


Semiconductors which are absolutely pure in state like Silicon and Germanium are known as intrinsic semi conductors. If its temperature is brought down a 0°K, it will act as a good insulator and very little current will flow through it. But at room temperature some electrons are thermally excited and they escape to conduction band and thereby account for their conductivity. Excitation is also possible with light, electrostatic, magnetic and kinetic energy.

When an electron becomes free from the atom of an intrinsic semi conductor, it breaks a covalent bond and leaves behind a vacancy called a hole. The hole in fact means the loss of an electron. The free electron and the hole form an electron hole pair. The higher the temperature the greater is the number of free electrons and therefore holes. The dots represent the electrons and small circle represents the holes. Suppose an electron is taken away from the bond 'A' as indicated, the bond is left which contains only one electron of two (pair). Suppose now the material is subjected to an electron field as indicated, an electron from a bond such as 'B' may then, under the influence of the field, tend to move to the vacant electron hole state in 'A'. This leaves a vacant electrostatic hole state in bond 'B and an electron from another bond such as 'C' may fill this hole.


By repeating the process we see that the change is transported under the influence of an electric field by virtue of the absence of an electron in an electron pair bond. It should be noted that the vacant electronic state moves in the same direction as would a positive charge carrier. For this reason these vacant electronic states are called as positive holes or simply holes. Thus at room temperature a number of bonds will be broken and conduction may be observed as a result of the motion of electrons and holes under the influence of an external field. For pure elements like Germanium and Silicon, the number of free electrons is equal to the number of mobile holes. Semi conductors of this type are called intrinsic semi conductors. They have negative temperature co-efficient.
Temperature Characteristics of Germanium and Silicon when used as intrinsic semiconductor is given in Table.


Germanium
Silicon
1. Carrier density
2 x 10-22T3/2
x 10-3700/T

2. Electron mobility
3.5 x 103 T1.37m2 / Volt sec
4.0  x 105 T-2.6 m2/ Volt sec
3. Hole mobility
9.1 x 104T-2.3m2 / Volt sec
2.5 x 104T-2.3 m2/ Volt sec
4. Electron of hole mobility at room temperature
0.38 m2 / Volt sec
0.17 m2 / Volt sec
0.18 m2 / Volt sec
0.035m2 / Volt sec

Extrinsic Semi Conductor:

When impurities are added to intrinsic semi conductors they become extrinsic semi conductors. The additions of impurities are called doping, and the extent to which it is added is called the level of doping. The impurities themselves are called dopants. The impurities change the energy band spectrum and hence the conductivity. The conductivity of extrinsic semi conductors can be varied over a wide range by varying the concentration of impurity. They impurities are of two kinds, namely materials having five valence electrons or materials having three valence electrons.

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